The semiconductor industry uses metal-containing interconnects, such as copper (Cu) and alloys thereof, in electronic devices such as, for example, state of the art microprocessors. The metal-containing interconnects, which may be embedded fine metal lines, form the three dimensional grid upon which millions of transistors at the heart of the microprocessor can communicate and perform complex calculations. In these and other applications, copper or alloys thereof may be chosen over other metals such as, for example, aluminum because copper is a superior electrical conductor, thereby providing higher speed interconnections of greater current carrying capability.
Interconnect pathways within electronic devices are typically prepared by the damascene process, whereby photolithographically patterned and etched trenches and vias in the dielectric insulator are coated with a conformal thin layer of a diffusion barrier material. A diffusion barrier layer is typically used in conjunction with a metal or copper layer to prevent detrimental effects caused by the interaction or diffusion of the metal or copper layer with other portions of the integrated circuit. Exemplary barrier materials include, but are not limited to, titanium, tantalum, tungsten, chromium, molybdenum, zirconium, ruthenium, rhodium, iridium, vanadium, and/or platinum as well as carbides, nitrides, carbonitrides, silicon carbides, silicon nitrides, and silicon carbonitrides of these materials and alloys comprising same. In certain processes, such as when, for example, the interconnect comprises copper, the diffusion barrier layer may be coated with a thin ‘seed’ or ‘strike’ layer of copper, prior to completely filling in the features with pure copper. In still other cases, the seed layer of copper may be replaced by—or used in addition to—an analogous cobalt or similar conducting thin film ‘glue’ layer. Excess copper may then be removed by the process of chemical mechanical polishing. Since the smallest features to be filled can be less than 0.2 microns wide and over 1 micron deep, it is preferable that the copper seed layer, copper glue layer and/or the diffusion barrier layers be deposited using metallization techniques that are capable of evenly filling these features, without leaving any voids, which could lead to electrical failures in the finished product.
Numerous methods such as ionized metal plasma (IMP), physical vapor deposition (PVD), chemical vapor deposition (CVD), atomic layer deposition (ALD), plasma-assisted chemical vapor deposition (PACVD), plasma-enhanced chemical vapor deposition (PECVD), electroplating, and electroless plating have been used to deposit a metal-containing layers such as copper, diffusion barrier, and/or other layers. Among the foregoing, vapor deposition methods, such as CVD and ALD using one or more organometallic precursors, may be the most promising methods for forming metal and/or metal-containing films because these methods provide excellent step coverage for high aspect ratio structures and good via filling characteristics, and better processing control over film thickness. In a typical CVD process, a vapor of a volatile organometallic precursor containing the desired metal is introduced to a substrate surface whereupon a chemical reaction occurs in which a thin film containing the metal as a compound or as a pure element is deposited on the substrate. Since the metal is typically delivered in a vapor form as a volatile precursor, it can access both vertical and horizontal surfaces to provide an evenly distributed thin film. In a typical ALD process, a volatile organometallic precursor is alternately pulsed into a reactor with a reagent gas such that self-limiting alternating monolayers of precursor/reagent are deposited on the substrate wherein the monolayers react together to form a metal film or a metal-containing film which is subsequently reduced to metal or used as deposited. For example, if a copper organometallic precursor was reacted with a suitable oxidant in an ALD process, the resulting cuprous oxide or cupric oxide monolayer or multilayer could be used for semiconductor applications or reduced to copper metal.
Vapor deposition processes such as CVD and ALD allow one to control thickness of the resulting film by controlling process conditions such as time and temperature and precursor and reagent flow and pressures. Typically, higher pressures, higher precursor flow and higher wafer temperatures may tend to provide thicker films for a given unit of processing time. For example, once a given ‘process window’ for a CVD process has been established, a rate of film deposition can be determined and, from that value, a particular film thickness can be deposited by selecting an appropriate deposition time for that process. Like the CVD process, once an ALD process has been established in which alternating pulses of organometallic precursor and reagent gas are cycled over the substrate to grow the metal film, the thickness of the film can be determined by controlling the total number of cycles used.
For certain applications, such as copper seed layers, it is desirable that the copper film be formed as thin as possible while still being continuous and unbroken. Since copper CVD processes tend to proceed by copper metal nuclei forming onto the substrate which slowly grow larger until the nuclei eventually touch each other to form a continuous film, the minimum film thickness achievable is governed by the point at which the copper nuclei coalesce. These nuclei grow larger by two processes. First, copper atoms which are deposited onto the surface of the barrier, seed, and/or glue layer migrate onto the nuclei to increase their size. Secondly, fresh copper atoms are grown directly onto the copper nuclei. Thus, copper is deposited both onto the diffusion barrier and/or glue layer as well as the copper already deposited.
A similar situation is encountered in copper ALD processes whereby a volatile organometallic copper precursor is alternately pulsed into a reactor with a reagent gas such that self-limiting alternating monolayers of precursor/reagent are deposited on the substrate such that the monolayers react together to form a copper film. In this way, copper can be deposited onto the barrier and/or glue layer and the existing copper surface or seed layer. The reference Zhengwen, Li, Antti Rahtu, and Roy Gordon, Journal of the Electrochemical Society, 153 (111) C787-C794 (2006) provides an example of a typical copper ALD process where additional copper is deposited onto an existing copper layer at a growth rate of 0.5 Angstroms per cycle. Thus, any copper nuclei, or larger localized areas of copper metal deposited onto a barrier or seed layer, can grow by surface copper atoms diffusing along the surface of the barrier material onto the nuclei or area or by fresh copper being grown directly onto the nuclei or area. Additionally, certain commonly used metals that are used for barrier layers such as tantalum may tend to promote copper agglomeration into a discontinuous film in a manner similar to that of water beading on a waxed surface (see, e.g., H. Kim, T. Koseki. T. Ohba, T. Ohta, Y. Kojima, H. Sato, Y. Shimogaki, Journal of the Electrochemical Society, 152(*), G594-G600 (2005)). Such a discontinuous copper film may lead to subsequent problems during copper electroplating. Thus, it is challenging to achieve a thin yet continuous copper film on a barrier, seed, and/or glue layer—including those layers containing ruthenium.
Accordingly, there is a need for a process wherein the copper selectively deposits onto a metal-containing barrier, seed, and/or layer in a reduced thickness or amount than that of the copper previously deposited.